The present invention relates to a signal processing device for transmitting and controlling an analog signal and for inserting a desired signal. In particular, the invention relates to a circuit which is ideal for use when an output signal corresponding to an analog signal is converted to a direct current voltage level during a desired period, or when a desired signal is inserted in an input signal transmission path.
As a device which is used to suppress temporarily the transmission of an input signal while the input signal is being transmitted is known a muting circuit. When, for example, a brightness signal of a video apparatus is muted, if the transmission of the brightness signal is simply suppressed, then the brightness of the reproduced video image is not constant so that the video image is hard to view. Also, as disclosed in Television Technology, July, 1989, p. 20-21, video apparatus have been developed in recent years having multiple functions. However, in such recent multi-function video apparatus, if a reference level during transmission of a signal, whether it is a brightness (luminance) signal or a color (chrominance) signal, is varied, then the brightness or color tone of the reproduced video image is caused to vary.
In view of the above-mentioned problem, for example, when the brightness signal is muted, then the brightness signal must be set as a d.c. voltage at a desired level, which requires an offset adjustment. On the other hand, because the video circuitry in recent apparatus is implemented with ICs, the offset adjustment must be performed by an adjusting element mounted externally to the IC. Such an arrangement takes much labor and long time for adjustment and also requires externally mounted parts, which results in increased manufacturing costs. Also, due to the fact that the externally mounted parts have a temperature dependence different from that of the circuit elements included within the IC, it is difficult to maintain a stable d.c. offset voltage when temperatures vary.
The present invention further relates to a signal processing device which produces output signals separately or in a mixed manner respectively corresponding to a plurality of input signals, and, in particular, to a circuit construction for use in stabilizing mixing of a plurality of signals in a signal transmission path in a video apparatus.
In recent years, there has been developed a video camera which is able to combine two picture screens, as disclosed in Television Technology, July 1989, pp. 25-26. Combination of two picture screens is achieved by means of an electronic volume control called a "linear gate" by the persons skilled in the art. With this circuit, the time for combining signals, designated a "MIX" time, can be adjusted by controlling the level of a control voltage.
In the conventional linear gate, there are provided a pair of differentially connected current switches in a transmission path for two input signals to be mixed, that is in a current path for the two input signals, so that an output signal can be obtained from a single load resistor. That is, in the linear gate according to the prior art, it is not possible to obtain two output signals which respectively have forward and reverse phases. In providing the video apparatus with multiple functions and simplifying the circuit arrangement of a multi-function video apparatus, it is very convenient to be able to provide two output signals having opposite phases.
However, according to the prior art structure, because only one output signal can be obtained, to obtain an output signal having an opposite phase, it is necessary to provide a phase inverter circuit, which results in a complicated circuit arrangement.
Also, according to the conventional structure, the pedestal levels of the input signals can easily be varied, the time for mixing the two input signals is short, and the structure can easily be influenced by power and temperature variations.